1. Field of Invention
The area of utility of the invention comprises prevention of development of system instability within power systems when threatened by transmission line faults, and certain other system stability endangering events.
The area of method comprises responding to faults, and other events that could endanger system stability, by rapidly initiating preprogrammed processes of
A. full or partial fast closure of intercept valves of steam turbine type generator prime movers of power systems, preferably effected within 1/4 second, PA1 B. subsequent partial reopening of intercept valves, preferably so effected that the valves begin to reopen somewhat in advance of occurrence of the peak of the first forward swing of the generator rotor, and substantially attain planned full extent of partial reopening within a fraction of a second following that peak. PA1 C. reduction of the steam acceptance of the high pressure turbine via control valve repositioning. PA1 d. preprogrammed slowly executed full opening of intercept valves subsequent to fast partial opening, PA1 e. runback of rate of steam production within the steam generator to a value that will cause termination of the discharge of steam through said power operated relief valves that will at first take place, with provision so that said runback is effected rapidly enough to avoid overheating of the reheater. PA1 A. valve closing in response to an electrical signal such as an indication of a line fault PA1 1. the post-fault or more generally the post stability endangering event driving power of the turbine or turbines down stream of the intercept valve or valves will exceed the pre-fault, or pre-event driving power, a circumstance which tends to adversely affect system stability, this being especially the case when the fault or other stability endangering event results in the sustained opening of one or more transmission system circuit breakers, and thereby operates to impede transmission of power in the post-fault or post-event regime, whereas actually it would normally be advantageous for the total post-fault or post-event driving power of the turbine to be held less than, and, as a rule, preferably somewhere in the range of 60 to 90 percent of pre-fault or pre-event value, PA1 2. reheat and MSR (moisture separator and moisture separator reheater) pressure safety valves may discharge steam and in some cases may thereafter leak and require maintainance. PA1 1. intercept valve closure which are fast enough and sufficient in extent to have a favorable effect on generator rotor first swing stability, and preferably take the form of full closure effected in 1/4 second or less, PA1 2. subsequent reopening of some or all intercept valves to a partially open position with reopening preferably initiated somewhat in advance of the first forward swing of the generator rotor, and carried to completion within a fraction of a second following the peak of that swing, PA1 3. preprogrammed servo valve implemented retention of partially opened intercept valves in, or substantially in, the preprogrammed position that it attained in its rapidly executed reopening process, PA1 1. full or partial closure followed by full reopening, PA1 2. full or partial closure followed by partial reopening, PA1 3. partial closure. PA1 a. fully closing all intercept valves in a fraction of a second by rapidly opening valve actuator oil dump valves, PA1 b. fully or partly closing all intercept valves under servo control, PA1 c. after initial full or partial closure of all intercept valves, repositioning under servo control, PA1 d. supplementing item (c) by fast partial intercept valve reopening initiated somewhat in advance of the generator rotor first forward swing, preferably effected within 1/2 second, with control of extent of reopening determined with use of metering cylinders or with servo or cam operated valves, PA1 e. fully closing all or some control valves in a fraction of a second by rapidly opening valve actuator oil dump valves, or PA1 f. fully or partly closing all control valves under servo control, PA1 g. after initial full or partial closure of control valves, fully or partly reopening under servo control PA1 h. supplementing item (g) by fast full or partial control valve reopening initiated somewhat in advance of the instant of generator rotor first forward swing, and preferably effected within 1/2 second.
These preprogrammed processes (a) through (c) may optionally be supplemented by providing power operated valves which are arranged to open and discharge high pressure steam either to atmosphere, or to the condenser, or to the cold reheat line, as a way to prevent discharge of steam through high pressure safety valves when and as control valve repositioning takes place.
2. Prior Art
My copending application, Ser. No. 388,619, now U.S. Pat. No. 3,848,138, covered the broad concept of combined employment of items (a), (b) and (c) above, coupled with
and also dealt with providing, in steam electric installations of fossil fuel type that are equipped to discharge steam to atmosphere or to the flash tank of once through boilers, and from there to the condenser, to effect coincidentally with items (a), (b), (c) and (d) a preprogrammed process of
In my copending application Ser. No. 506,441 I note that alternate steam by-passing techniques could be employed (cf page 30), but I did not discuss in detail what would be involved.
It is old in the prior art to provide, in steam electric plants, power operated valves which are arranged to open in response to an increase in pressure ahead of the valve and discharge steam, either to the condenser, or to the steam line that receives the discharge of the high pressure turbine, and from the hot reheat line to the condenser, as a way to allow responding to a sudden reduction of station export load by rapidly partly closing turbine valves, without causing lifting of and possible damage to safety valves, but, as far as I know, I was the first to propose employment in steam electric plants equipped with by-pass systems of these types, of provision for sustained reduction of turbine driving power type fast valving as a way to favorably effect power system stability when endangered by transmission line faults, or other stability endangering events.
The novelty of the present invention relates principally to providing for stability endangering event initiated fast valving of the sustained reduction of driving power type, in steam electric installations that are equipped with such by-pass systems.
Additionally novelty resides in providing, in such installations, for use of techniques of rapid sustained reduction of turbine driving power that are adapted to favorably effect the transmission circuit and station loading that can be safely employed despite the occurrence of severe events that endanger stability.
First published prior art in the area of fast prime mover driving power control for stability improvement came in 1929 (1), (Numbers in parentheses refer to Table of References), and the first published description of a successful reduction to practice in 1931 (2).
U.S. Pat. No. 1,935,292, which issued in 1933 (3), in which a Pelton wheel was shown as the source of driving power, provides a good picture of early control concepts, which were based only on momentary valve closure.
Reference 4 is mainly of interest in that it had the effect of deflecting interest from both fast turbine valving and momentary application of braking load.
Reference 5 which was published in 1934 cited the findings of reference 2, and led to a sequence of USSR studies of fast steam turbine valving for system stability improvement (5,6,7,10,11,12,13,17), and one U.S. patent (18).
In 1962 reference 21 introduced the concept of providing to rapidly bring into effect a sustained partial reduction of turbine driving power as a means of favorably affecting system stability when threatened by a line fault. However, though, as it happened, at the point in time when the application that included it was first filed, the concept was not only novel, but, as events have shown, non obvious, claims were limited to varying the nature of a preprogrammed process of turbine valving in response to the extent and distribution of prefault transmitted load.
In 1966 reference 22 which was reissued in 1969 (23) introduced the concept of employing fast valving of the sustained reduction of driving power type, and momentary application of braking load, in combination, and of preprogrammed response to the post-fault event of unsuccessful reclosure of faulted line circuit breakers.
Beginning in 1961 the writer undertook to solicit interest on the part of power producers and turbine generator manufacturers in fast turbine valving optionally supplemented by momentary application of braking load.
Meetings were held with GE and Westinghouse engineers and, following the 1965 Northeast U.S. blackout, GE engineers presented a paper on fast valving at the 1966 American Power Conference (24).
This 1966 GE paper stimulated interest in momentary type fast turbine valving on the part of a number of U.S. power producers, and led to the publication of several papers (25,26,27,28,29,30).
In February 1966 the writer filed a patent application which issued as U.S. Pat. No. 3,515,893 (31), and subsequently filed for reissue of this patent (32) and in 1968 presented a paper at the American Power Conference (33) which dealt in large measure with what had been set forth in the application for the patent, which included, among other things, sustained type fast turbine valving, used alone or in combination with momentary application of braking load.
As early as April 1964 the writer began to look into what was available in the way of power operated relief valves both that could be used to divert or dump high pressure steam to atmosphere or the condenser as a way to prevent discharge of steam through high pressure safety valves when high pressure turbine steam acceptance was reduced by closure of control valves, and in reference 31 cited the concept of programming initiation of the opening of such valves as an aspect of fast valving control procedures (as see reference 34).
A 1966 review of USSR references revealed that, as of 1965, in the USSR, certain once-through steam-electric installations were equipped with fast acting valves that would by-pass steam around the high pressure turbine to the cold end of the reheater as control valves closed, thereby avoiding both safety valve lifting and boiler problems, and also valves that, at the same time would by-pass steam from the hot end of the reheater to the condenser, (14, 15) using, for the purpose, valves of a special type that incorporated provision for desuperheating and that, as it later appeared, were at that time being provided in once-through type boilers of German steam electric installations where they were used during startups and to make possible rapid speed responsive reductions in unit load (35, 39). This type of valving, therefore, represented one approach to a solution of the problem of safety valve lifting though not an approach that could easily be applied in U.S. type steam-electric installations.
In the fall of 1966 the writer proposed, in a meeting with engineers of Southern California Edison, that provision be made for sustained type fast valving and momentary dynamic braking of 795 mw steam-electric units nos. 4 and 5, of the Four Corners steam station as a way to preserve system stability on loss of the single circuit 500 kv line leading across Arizona to a 500 kv SCE substation located near the California-Arizona border.
This led, in due course, to joint Southern California Edison and Arizona Public Service system studies which were reported on in a 1968 AIEE paper (25), and to an SCE decision to specify that the turbines in question would be equipped to allow employment of sustained type fast valving.
In his 1968 paper (33) the writer made the point that steam by-passng could be used to prevent safety valve lifting, and for other purposes, including protection of reheaters, and also made reference to rapid fuel reduction as a way to avoid excessive excursions in temperature of superheaters and reheaters and, in these connections, cited a group of references including the above cited USSR references and also US papers dealing with rapid reduction of fuel supply.
As of 1966 the writer had run into the fact, that whereas in once through type boilers it is practice to provide by-pass valves to the flash tank of the boiler, for the purpose of discharging water to the flash tank during the startup process, the point applied that when these valves are opened under load conditions, steam passes through them to the flash tank and thence to the condenser, which suggested that it might be that, since Babcock & Wilcox's once through boilers are equipped with fast acting (air operated) type valves that by-pass the primary superheater, fast initiation of their opening might operate to prevent safety valve blowing on fast closure of turbine control valves, or, at any rate, that this would be the case if fast closing was promptly followed in sufficient degree by fast partial reopening.
The fact that fast opening of the bypass valves in question could prevent safety valve lifting was eventually confirmed, when, in 1969, tests on a 600 megawatt steam-electric unit incorporating a Babcock & Wilcox oil fired boiler demonstrated that with use of preprogrammed fast opening of the primary superheater by-pass valves it was possible to trip the unit at full load without blowing safety valves, and, by, at the same time, reducing oil supply to 10 percent in 8 seconds, after another minute or so, resynchronize and begin to reload, and fully reload in 20 minutes (37).
Moreover unpublished information which the writer had been furnished by the GE in 1966 (38) was to the effect that a 50 percent sudden step reduction of turbine driving power would not cause significant damage to fossil fuel type turbines. Also information was obtained from Babcock & Wilcox to the effect that even for the case of coal firing, a 40 to 50 percent reduction in fuel feed could be achieved rapidly enough to avoid development of excessive reheater and superheater temperature.
Putting all these facts together made it appear that fast turbine valving of a type that would effect no more than a 40 to 50 percent sustained step reduction of the steam acceptance of a high pressure turbine of any fossil fuel steam-electric installation, whether or not coal fired, would represent an entirely feasible thing, and that in the case of Babcock & Wilcox once through boilers already equipped with fast acting by-pass valves, also something that would involve little expense.
This information was communicated to the Tennessee Valley Authority, with which the writer had and presently retains a consulting arrangement, and eventually led to a decision to apply the scheme to unit No. 2 of TVA's Cumberland generating station (36).
The information contained in reference 37 was also furnished to the Chief Mechanical Engineer of Southern California Edison, in the context of employment of the principle of fast opening of the by-pass valves to the flash tank of the Babcock & Wilcox boilers of two jointly owned 800 mw steam-electric units located in the Four Corners, New Mexico plant of Arizona Public Service, as a hitherto unrecognized as feasible, very simple way to provide the steam by-pass capability needed to allow fast valving these units to half load, whereby to avoid need to trip-off one unit in response to the occurrence of a fault on the 500 kv line leading from Four Corners to Los Angeles and do so while avoiding lifting of safety valves.
However, though, in 1971 there was presentation of a technical paper (67) authored by engineers of the Los Angeles division of Bechtel Corp., which dealt with provision, at Four Corners, for use of fast valving of the sustained reduction of driving power type, achieved with combined use of preprogrammed sustained partial control valve closure and fast runback of steam generation, this paper makes no reference to providing so that by-passing of high pressure steam to the condenser, via the flash tank, will take place when, under heavy load conditions, a line fault would cause fast valving to come into effect.
Also the writer supplied a copy of reference 37 to a Working Group of the System Design Task Force of the New England power systems, that had been directed to carry out a study of fast valving. However in their report of their studies, though the Task Force discussed what was disclosed in reference 37, this was done only in the context that it demonstrated feasibility of tripping turbine generators of once through steam-electric units to auxiliary load and rapidly resynchronizing and reloading (40).
Again, though the report noted that performance advantages that could result from providing to close control as well as intercept valves, it was stated that, in the case of fossil fuel units, the result would be lifting of boiler safety valves, combustion and feedwater control problems, turbine thermal stresses, transient thrust unbalance, and overloading of the last few high pressure stages.
The report goes on to add that a turbine by-pass system closely coordinated with the turbine control valves could alleviate the problems cited, but would add cost and complexity, following which statement no further reference is made to control valve closure, except the statement having reference to nuclear turbines "As with a fossil unit, fast valving of the interceptor valve is employed as the most practical procedure."
An aspect of the report that bears on the obviousness of claimed subject matter of the present invention is that the report fails to make any reference to the fact that it is possible to prevent lifting of safety valves, merely by providing fast acting power operated relief or by-pass valves which do not have to be coordinated with the turbine's control valves, but rather, as provided for in the present invention, would be arranged to open and close in response to changes in steam pressure.
The above and other information that could be cited suggests that it was not obvious to the power systems engineers involved that in the case of fossil fuel and nuclear steam supply systems of U.S. type there were viable ways to employ fast turbine valving of a type in which a program of rapidly executed control valve sustained partial closure would be made use of as a way to prevent loss of system stability and avoid need to trip-off generation, while avoiding lifting of safety valves, and that, in the case of Babcock & Wilcox once through type boilers, this could be done without need to provide by-pass valve capability beyond or different from that already available.
Moreover non obviousness of the feasibility of safely employing fast control valve closure either on a momentary or sustained partial closure basis is confirmed by reference 24, which, though it cited the start-up by-pass system, did so only in the context of its function as something adapted to protect the boiler, rather than as a potentially effective means of preventing lifting of safety valves (41), and also by reference 29 which advises that the GE stand is in opposition to employment of fast reduction in high pressure turbine steam acceptance via "fast action in the control valves as a supplement to fast closing of intercept valves" and cited by way of explanation "possible repercussion on the boiler safety valves and boiler control" (42), a view that is also expressed in reference 27 (43).
Thus U.S. engineers have shied away from employment of fast control valve repositioning as an aspect of fast turbine valving for system stability improvement based on the opinion that problems that could not be readily solved stood in the way, and for this reason have favored restriction in the scope of fast valving precedures notwithstanding the fact that, on the basis of this policy benefits that fast valving could provide when control valve repositioning is employed could not be realized.
The present invention shows how penalties that were visualized as applying when control valve repositioning is employed as an aspect of fast valving for system stability improvement can in some cases be gotten around with the benefit of only minor changes in boiler and turbine control systems and methods, and how also, where this is not feasible, costs involved in providing needed fast acting steam by-pass capability can be minimized.
When it comes to the changes in turbine control measures proposed in the present invention these include the concepts of rapidly partially reopening control and intercept valves.
In this connection the concept of merit in rapid reopening of intercept valves, with initiation of opening occurring at or in advance of the first forward swing of the generator rotor, appears to have been first publicly noted as desirable in reference 13 and was also noted in references 46, 47 and 49. However as going beyond what is put forward in those references the present invention calls, as suggested in reference 48 and 50, for at first rapidly opening intercept valves part way, and thereafter reopening them more slowly, and goes on to explain how this objective can be implemented.
While not a matter of public record it may be worthy of note that, as was brought out in a memo prepared by the writer and dated March 11, 1964, copies of which were sent to Westinghouse engineers, the desirability of providing for the fast partial lifting of intercept valves was put forward in a Mar. 5, 1964 meeting with Westinghouse engineers and a representative of Westinghouse's Patent Dept.
In the memo the writer stated that his "fast governing" concept "would ideally call for" among other things
a. rapid closing of control and intercepting valves, as in a matter of 1/4 second, followed by timed initiation of reopening, at the same rate, after a time interval in the range 1/8 to 3/4 second from the initiating signal, PA2 b. reopening at equal speed to a controlled valve opening which could correspond to a load equal to or lower than that prevailing prior to the initiation of closing operations.
Here the idea of providing for a variation in dead time was to allow for the fact that, as is well known to system planners, in some situations generator rotor swings peak as in 0.4 seconds, and in other cases, more slowly as in as much as 1 second, or more, with the effect that where slow enough peaking can be expected, a 3/4 second delay in intercept valve reopening can be useful.
Another novel use of control systems that is disclosed in the present application comprises the technique of closing some but not all of the control valves of a partial admission type high pressure turbine, at top speed, with the use of valve actuator oil dumping, as a simple way to provide for very fast partial reduction of the steam acceptance of turbines of that type.
In the light of the foregoing the rendering feasible of employment of fast momentary full and sustained partial control valve closure by the methods which the writer has briefly outlined, and which are characterized by the fact that they are well adapted to implementation in the context of U.S. designs of steam-electric installations, as also the improvement of effecting fast only partial control and intercept valve reopening, which include among their advantages prevention of development of loss of synchronism on generator rotor swings subsequent to the first, are viewed as novel and non obvious, and moreover the same is also thought to apply to the concept of effecting a sustained reduction of the driving power of turbines of BWR nuclear and other steam-electric installations by delaying full opening of intercept valves while tolerating discharge of steam through low or intermediate pressure safety valves over a period during which rate of generation of steam within the steam supply source is in process of being reduced on a preprogrammed basis.
In relation mainly to the prior art of fast valving taken as a whole, it may be in order to, at this point, take note of reference 18 comprising U.S. Pat. No. 3,421,014 which issued on Jan. 7, 1969.
This patent describes methods of fast valving steam turbines for system stability improvement which operate via employment of a feed-back type of control system, rather than a system of feed-forward type which characterize fast turbine valving techniques disclosed in the writer's patents and pending patent applications.
Also the patent in question makes no reference to steam by-pass systems, or to fast control of rate of steam generation within steam supply sources, even though in references 8, 9, and, apparently, also in 10, the inventor describes by-pass valve arrangements that allow discharging entrained steam to the condenser through the turbine's intercept valves.
From the above it does not appear that the patent anticipates what is set forth and claimed in the present application, while also this applies, as well, to what is set forth in references 8, 9 and 10.
Another point that may be worthy of note is that while in German once-through boiler type steam electric installations which incorporated what can be termed German type by-pass systems provision was made for simultaneous fast closure of control and intercept valves, as in the case of U.S. Pat. No. 3,421,014 which has relation to USSR practice, valve repositioning was brought into effect on a feed-back basis, in response to speed increase, rather than in a preprogrammed manner adapted to cause valve position changes to take place independently of speed change.
Also it may be worthy of note that, in the period 1966 through 1968 the writer discussed the concept of rapidly effecting preprogrammed sustained reduction in the steam acceptance and driving power of turbines that would be equipped with German type by-pass systems, in meetings with engineers of the consulting firm Ebasco Services, with engineers of Combustion Engineering and with the Chief Mechanical Engineer of Southern California Edison.
However, in the U.S., this type of by-pass system though at one time given a trial in a Combustion Engineering boiler was not viewed as worthy of use in U.S. installations based on the fact that expense would be high and that the valves could leak.
In 1969 the Chief Electrical Engineer of Ebasco Services helped to arrange so that the writer could meet at the 1969 American Power Conference with the author of reference 35, a representative of Siemens, and at this meeting and subsequently the writer endeavored to interest Siemens in offering provision for preprogrammed fast valving to potential turbine-generator purchasers.
Later at the 1971 ASME-IEEE Joint Power Generation Conference the writer contacted the author of reference 65, and also representatives of the German turbine generator manufacturing firm of M.A.N. who were attending the meetings.
These contacts with Siemens and M.A.N. personnel were supplemented by correspondence, and the transmittal to these firms of copies of references dealing with fast turbine valving, of preprogrammed type, as a way to improve system stability, but this activity on the part of the writer elicited no evidence of action when it came to dealing with potential customers.
This presumably, was due in part, to the fact that, whether in the U.S., or Germany, the subject of power system stability, typically, has hardly at all been understood by mechanical engineers of turbine generator manufacturing establishments.
At the present point in time the potentiality of momentary type fast valving for stability improvement is beginning to become clear to Europeans, as see in this connection, sections 3.4, 3.5 and 5.8 of reference 66, and it will certainly be obvious to Europeans that fast closing and opening of control and intercept valves can easily be provided where German type by-pass systems are used.
Also in U.S. high temperature gas cooled, or HTGR, nuclear installations, German type by-pass systems are used, and it will be obvious how to apply preprogrammed fast control and/or intercept valve closing in these installations, as a way to improve power system stability.
However, though it would be obvious, in the light of the prior art, how to provide to fast open valves subsequent to fast closure, there appears to the writer, to be no evidence that, in reference 66, the position is taken that it would be desirable to rapidly reopen intercept valves part way and thereafter more slowly, or, for that matter, that it would be desirable to hold control valves in a partly open position on a sustained basis.